For some applications there are requirements that the enclosure for electrical apparatuses is able to withstand an internal explosion of flammable gas or vapour that may enter it, without suffering damage and without communicating the internal flammation or sparks to the explosive atmosphere through any joints or structural openings. This is obtained by providing flame paths of sufficient strength and integrity to withstand internal explosions without communicating the internal flammation to the external flammable gas or vapour through these joints.
With pumps, one such structural joint is the passage of the drive shaft through the housing. One example of providing flame paths is to arrange a gland surrounding the pump shaft, which is mounted in the wall separating the motor side from the impeller side. The gland is designed as a metal cylinder surrounding the shaft arranged to provide flame paths between the shaft and the gland. The gland has a rather tight fit around the shaft. At the same time there must be space enough to form a film but not so tight that there is a risk for metallic contact. There is especially a risk for metallic contact when there are loads on the impeller causing the pump shaft to bend, in particular when the shaft protrudes quite a distance from the journal point.
The fixed mounting of the gland in the wall means that when the shaft bends there is a risk that it will come in contact with edge parts of the gland. This problem is even more pronounced with pumps that are intended to be used in an environment where explosive gases are present where requirements stipulate that the gap between the gland and the shaft has to have a minimum width and minimum length in order to ensure that no sparks from the motor can enter through the gap.